When an operation in a heat transfer tube of a heat exchanger or an evaporation tube of a boiler facility or the like is performed for a long period of time, a change in the diameter of the tube occurs.
(1) For example, since an evaporation tube for a land boiler is exposed to a reduction atmosphere so as to suppress NOx in a flue gas in a combustion atmosphere, a problem arises in that corrosion occurs due to the adhering of sulfide.
(2) Further, for example, in a heat exchanger for desulfurization equipment, a problem arises in that corrosion occurs due to the adhering of sulfide produced in a desulfurization process to a heat transfer tube.
(3) For example, in the heat transfer tube of the heat exchanger, a problem arises in that a thickness reduction occurs in the inner diameter due to the abrasion of water when the heat transfer tube is used for a long period of time.
For this reason, a related art has proposed a configuration in which corrosion is inspected by an ultrasonic sensor or a flaw detecting sensor using a high-frequency eddy current (see Patent Literatures 1 to 3).